This invention relates to a method of controlling occurrence of severe slugging in a riser of a pipeline conveying a multiphase fluid system, and also relates to a combination comprising a pipeline with a riser for conveying a multiphase fluid system wherein the combination is adapted to control occurrence of severe slugging in the riser.
When natural gas is taken from where it occurs in nature in a naturally occurring gas reservoir in the earth""s crust by means of a well supplying a pipeline from a well-head, the supplied gas is often naturally accompanied by liquid, for example water and/or hydrocarbon liquid. Such hydrocarbon liquid may be or may comprise oil. Thus the pipeline conveys a multiphase fluid system to a production facility which may comprise separator means to separate the gas from the liquid, gas drying means, filtering means, cooling means, and dewpointing means etc. Frequently a substantially vertical riser connects the pipeline with the production facility; this is particularly the case where the well is under water, for example under the sea or a lake where the pipeline can be on a bed of the sea or lake from which bed the riser ascends, often through a considerable distance, to the production facility, which is usually above the surface of the water on a production platform. That platform may be unmanned and may be in a remote and/or hostile location. If the gas flow is above a certain rate the multiphase system ascends through the riser in a churn flow of a mixture of the gas and liquid. But when the gas flow is slow the liquid phase can form one or more slugs of liquid at a base of the riser and eventually the liquid slugs increase in size or combine to an extent which blocks off the riser thus stopping the flow of gas to the production facility. This is the commencement of severe slugging. Gas pressure upstream of the blocking slug increases pushing even more of the forming liquid slug into the riser so that the head of the slug in the riser ascends towards the upper end of the riser. The column of liquid slug creates a hydrostatic pressure which increases as the column lengthens, and this pressure is substantially equal to the increasing gas pressure in the pipeline. A stage is reached where the upper end of the liquid slug discharges into the production facility upon the riser becoming substantially full of the slug. Now the hydrostatic pressure is a maximum for the riser and liquid concerned, and the gas pressure downstream of the slug forces a bubble of gas into the lower end of the riser which immediately reduces the hydrostatic pressure exerted by the reduced length of slug up the riser. The excess of gas pressure over the hydrostatic pressure causes the slug to shoot up the riser at high speed followed by a sudden rush of gas which all threaten to overwhelm the production facility. This blowdown is detected by severe slugging detection means which operates to cause valve means to close to cut off the riser from the production facility and also close down operation of the latter. This means production of gas is stopped whilst the effects of the severe slugging are dealt with, and production may not be resumed for at least several hours thereby causing financial loss which can be exacerbated by the possibility of resumed production having to be at a low level and then progressively increased to a normal rate.
Severe slugging can be a cyclical phenomenon.
An object of the invention is to provide a method of controlling occurrence of severe slugging by intervention in operation of the pipeline to prevent occurrence of said severe slugging.
According to a first aspect of the invention there is provided a method of controlling severe slugging in a riser of a pipeline conveying a multiphase fluid system, the method comprising providing separation vessel means connected to an upper part of the riser to receive therefrom the multiphase fluid system for separation of a gas phase of the fluid system from a liquid phase, providing the separation vessel means with a gas outlet comprising valve means to regulate speed of gas flow along the pipeline towards said riser in at least a vicinity of a lower end of the riser, observing pressure in the pipeline at position adjacent to said riser, and varying an extent to which said valve means is open so as to vary gas velocity in the pipeline to a value opposing or preventing occurrence of severe slugging in the riser.
The observed pressure may be gas pressure.
The extent to which the valve means is opened may increase the velocity of the gas flow in the pipeline adjacent to the riser.
The extent to which the valve means is opened may be increased when the observed pressure rises above a pre-determined value. Said pre-determined value may be derived empirically.
The extent to which the valve means is opened may be a function of a three term control.
The valve means may be operated with a view to maintaining the observed pressure in the pipeline at a substantially pre-determined value.
According to a second aspect of the invention there is provided a pipeline to convey a multiphase fluid system, said pipeline comprising a riser, separator vessel means connected to an upper part of the riser to receive therefrom the multiphase fluid system to separate a gas phase of the system from a liquid phase, said separator vessel means being provided with a gas outlet comprising valve means to regulate flow of gas from the separator vessel means, pressure observing means to observe pressure in the pipeline adjacent to said riser and provide a signal corresponding to observed pressure, control means responsive to said signal to cause said valve means to operate to vary an extent to which the valve means is open, and the arrangement being such that when the pressure observing means observes a pressure greater than a pre-determined value said control means causes operation of the valve means to vary the extent to which the valve means is open so as to vary gas velocity in the pipeline to a value opposing or preventing occurrence of severe slugging in the riser.